Biaxial negative C-plate retarders are suitable for use as compensators in LCDs to improve the optical parameters such as contrast ratio and grey scale representation at large viewing angles. A biaxial negative C-plate retarder in its optical properties approximates to a combination of a planar A-plate and a negative C-plate, but shows better optical performance than such a combination. The in-plane anisotropy of the biaxial negative C-plate retarder (Δnxy) approximates to the A-plate and the out-of-plane anisotropy (Δnxz and Δnyz) to the negative C-plate. Simulations have shown that the optical performance of the biaxial negative C-plate retarder is superior to that of the A-plate and negative C-plate stacked sequentially, and shows exceptionally good viewing-angle performance for LCDs. Furthermore, the use of a single biaxial film instead of two stacked films reduces costs and manufacturing problems.
WO 03/01544 discloses a biaxial negative C-plate comprising a cholesteric film having a deformed helix structure with an elliptical refractive index. It also discloses a method of preparing such a film from a polymerisable cholesteric LC material comprising a dichroic UV-photoinitiator. The cholesteric material is applied onto a substrate and irradiated with linear polarised UV light. The use of a dichroic UV-photoinitiator in combination linear polarised UV light induces a deformed helix structure in the LC material which is then fixed by in-situ polymerisation. As a consequence the resulting polymer film exhibits an elliptical refractive index and optical biaxiality.
However, the films disclosed in prior art do only have a limited biaxiality. To increase the biaxiality of the films new materials and methods are required.
It was an aim of the present invention to provide new materials and methods for preparing biaxial negative C-plate retarders with improved properties, in particular with a higher biaxiality index. Another aim of the present invention was to extend the pool of materials for preparing biaxial films available to the expert. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
It was found that these aims can be achieved by providing materials and methods as claimed in the present invention.